1. Field of the Invention
This invention pertains to package (i.e., “shop-assembled”) boilers for generating steam and, more particularly, to such boilers which have one or more lower and upper drums, with tubes connecting the drums for natural circulation (i.e., without circulation pumps) of the water, steam, and mixtures thereof within the tubes and drums. Such boilers may be fired by natural gas, oil, coal, wood, municipal solid waste, or refuse-derived fuel. (The term “boiler”, as used herein, is not limited to a vessel which contains fluid. The term embraces an entire steam generating apparatus, and could include for example, in addition to vessels for containing water and/or steam, a furnace, tubes, drums, pipes, ducts, hardware, and accessories.) A lower drum is a “mud” drum, which is sometimes called a “water” drum. An upper drum is a steam-and-water drum or “steam” drum. Package boilers are assembled in a factory and, unless they are to be transported by water, have a weight and exterior dimensions that comply with the maximum limitations for shipment by truck and/or railcar. Field-erected boilers, in contrast with package boilers, are assembled at the operating site from components and materials shipped to the site.
2. Description of Related Art
Boilers are required in many applications, worldwide, for the production of saturated and superheated steam. Saturated steam is primarily used for process steam, heating, and air conditioning, whereas superheated steam is primarily used in steam turbine applications for the generation of electrical power. A typical boiler consists of three heat absorbing sections: a radiant section, a convection section, and possibly a superheater section. The radiant section is the heat transfer surface area exposed to the flame in the combustion chamber (sometimes called the “furnace” or the “firebox”), whereas the convection section is the heat transfer surface area shielded from the flame, but excluding any superheater. The radiant section and the convection section each comprise riser tubes connecting the mud drum to the steam drum. The superheater section is located in either the high temperature section of the boiler convection section or at the rear of the boiler. Saturated steam generated in the radiant and convection sections of the boiler is collected in the steam drum and then, at the saturation pressure and temperature, is transferred to the superheater section of the boiler, where it is raised to a higher temperature while remaining at approximately constant pressure.
The oxidization of a fuel in the combustion chamber of a boiler generates thermal energy that is transferred to the water in the various sections of the boiler and thereby creates the steam that is generated inside the radiant and convection sections. The efficient conversion of the energy in the fuel into steam is of practical importance, as is the cost of the boiler system.
Package boilers are described in various publications and patents. These include Steam/Its Generation and Use (40th edition), published in 1992 by The Babcock and Wilcox Company, at pages 25-8 and 25-9; Combustion Fossil Power (4th edition), published in 1991 by Combustion Engineering, Inc. (especially pages 8-36 to 8-42); Keystone Steam Generating Systems, a brochure published in 1978 by Zurn Industries, Inc. (especially pages 1 to 7); Type A Boiler, a brochure published by The Bigelow Company of Madison, Conn.; Dang U.S. Pat. No. 5,881,551 (especially column 1, lines 35 to 65); and English U.S. Pat. No. 6,901,887. The English patent, for example, discloses a package water tube boiler which has a steam drum and a mud drum disposed within a single enclosure that is transportable by a single truck or railcar. A gas- or oil-fired burner fits within an opening in the front end wall. As best shown in FIG. 2, the combustion gases flow in a U-shaped pattern. The boiler has a furnace or combustion section separated from a convection section by a membrane or curtain wall which serves as a baffle. The gases flow toward the rear end wall (i.e., upward on the page), take a left turn through a furnace exit passage, take another left turn, head back through the convection section toward the front end wall (i.e., downward on the page), and then take a right turn and exit through an outlet. In the furnace the gases heat water and water-steam mixtures in radiant tubes by radiation, while in the convection section the gases heat water in convection tubes by convection. Other patents relating to package or small boilers are Shimer U.S. Pat. No. 2,187,632; Rehm U.S. Pat. No. 2,322,390; Marshall U.S. Pat. No. 2,763,243; Marshall U.S. Pat. No. 2,823,651; Vorkauf U.S. Pat. No. 2,988,063; Winship U.S. Pat. No. 3,266,467; Russo U.S. Pat. No. 3,543,733; Gerst U.S. Pat. No. 3,465,726; and Sharan U.S. Pat. No. 3,608,525.
A fundamental advantage of package boilers is an installed cost which is considerably lower than that of a field-erected boiler. This cost advantage is made possible by a number of factors. These include basic designs that allow standardized processes for fabricating multiple units in a factory, while providing sufficient flexibility to permit straightforward, efficient, and technically sound adaptation to the specific needs of a particular application. Another advantage is quality and reliability made possible by manufacture under controlled conditions by skilled and experienced personnel.
The following table summarizes current working limitations for shipment of package boilers in the U.S. by truck and railcar:
LengthWidth HeightWeightTruck480 in192 in180 in130,000 lbsRailcar540 in150 in198 in300,000 lbs
In recent years the number of advantages of package water tube boilers over field-erected boilers, and the growing magnitudes of some of these advantages, are responsible for their increased popularity and use of such boilers in the United States, and has resulted in their being called upon for more demanding applications. In response, innovation has been concentrated in the structure and design geometry of each new package water tube boiler design. English U.S. Pat. No. 6,901,887, for example, discloses a novel offset drum arrangement and water tube location, path, and configuration, in order to increase the heat transfer surfaces of the steam drum and the radiant tubes to increase boiler performance and efficiency, while shielding the drum from substantially all the flame in the combustion chamber. This serves to maximize the heat transfer area for a given total boiler volume (as determined by its exterior dimensions), and ultimately the steam generation capacity (as measured for example in pounds per hour) for that volume. This patented design also allows optimization of the width and height of the combustion chamber by making them approximately equal, so that the transverse cross-section of the combustion chamber is approximately square, thereby enabling the combustion chamber to better conform to the shape of the flame, facilitating control, and reducing NOx emissions.
Such structural and geometric innovations in design may not continue to solve the problem of meeting the increased demands, however. The operating parameters of package water tube boilers subject to the demands are approaching asymptotically the unyielding design limits imposed by the requirements of truck and rail transportation. The closer the designs approach these limits, the more they lose their flexibility, and specifically their ability to accommodate other, application-specific design requirements.
Consequently, field-erected boilers have been chosen instead of package boilers for many of these applications. While some demanding applications may permit the use of a number of conventional package boilers in combination, this requires unnecessary duplication and is likely to prevent the designer of the package boiler from taking full advantage of economies of scale, which in turn imposes penalties of cost and efficiency on the package boiler, relative to the field-erected boiler in competition with it.
An example of a demanding application that does not permit the use of conventional package boilers in combination is a boiler that must have a combustion chamber with a large internal volume and a burner with a large transverse cross section in order to keep emissions at desired levels. Such applications typically mix fuel, air, and exhaust gases over a large volume in order to reduce the formation of NOx gases. They have been required for boilers used in processes that produce ethanol.
An example of a demanding application that does permit the use of conventional package boilers in combination is one that merely has a requirement for steam capacity, either continuous or peak demand, that exceeds the steam capacity of a single, conventional package boiler. In such applications the package boilers are typically connected in parallel or assigned to a portion of the load.
The general concept of a package boiler assembled from two modules, each of which is transportable by rail or road, is known. In 1976 Csathy U.S. Pat. No. 3,971,345 disclosed a coal-fired package boiler assembled from two such modules—a furnace module and a convection module. The furnace module was stood on its end and bolted to the convection module at the operating site, in order to provide a large, tall, vertical furnace, which was necessary to address problems such as incomplete combustion of the carbon particles in the furnace module and excessive slagging in the convection module. Similar arrangements have been disclosed in commercial literature as “package”, “shop-assembled”, or “modular”. Examples are the brochure Nebraska Coal Fired Water Tube Boilers published by Nebraska Boiler Company, Inc.; the brochure A Better Boiler . . . By Design, published by Abco Industries, Inc., and the information sheet Modul-Pak II-Model WWF, published by Industrial Boiler Co., Inc. These appear to require more elaborate on-site assembly/construction than Csathy. Conceptually, while Csathy and these commercial designs separated boiler components into modules that can be transported separately, I do not believe that they were seeking to design an efficient, flexible system of modules from the ground up. None of them, for example, discloses a furnace module having a mud drum, a steam drum, any serious steam-generating capacity, or any steam separation apparatus.